Nitrogen Source and Rate Effects on Irrigated Corn Yields and Nitrogen‐Use Efficiency

Nitrogen rate studies were conducted under furrow‐irrigated corn ( Zea mays L.) production on a silty clay soil to compare granular urea with polymer‐coated urea (PCU) and stabilized urea (SU, contains urease and nitrification inhibitors) effects on corn yields, plant N uptake, and N use efficiency. Polymer‐coated urea had a yield advantage over urea 2 (continuous corn) of 3 yr at N rates below maximum yield, which resulted in greater economic returns with PCU (4–14%) at N rates from 168 to 280 kg N ha −1 . The SU fertilizer had no yield or economic advantage over urea. Grain and stover yields and N uptake increased with increasing N rate for all N sources. Expressing grain yields from all N sources as a linear‐plateau function of N rate showed that yields were maximized at 14.3 Mg ha −1 at an N rate of 254 kg N ha −1 or available N level (soil NO 3 –N plus fertilizer N) of 295 kg N ha −1 . Nitrogen recovery efficiency (RE) tended not to vary with N rate, with no differences between SU and urea but greater RE (19%) with PCU than urea under continuous corn. Fertilizer N use efficiency did not vary with N rate but was greater for PCU (36%) than for urea (32%) under continuous corn. In contrast to SU, PCU provided grain yield and potential economic advantages over urea under continuous corn production at N rates below those needed with urea for maximum grain yield.